Electrodeposition of copper from sulfur-free cyanide electrolytes using periodic reverse current

ABSTRACT

A PROCESS FOR ELECTROPLATING COPPER ON A BASE MEMBER COMPRISING APPLYING TO THE MEMBER AN ESSENTIALLY SULFURFREE AQUEOUS ALKALINE CYANIDE PLATING BATH COMRPISING A SOURCE OF COPPER, AN ACETYLENIC ALCOHOL, A COMPLEXING AGENT AND A HYDROXY ACID. CURRENT IS CAUSED TO FLOW THROUGH THE MEMBER WHILE IN CONTACT WITH THE PLATING BATH FOR A PERIOD OF 20-60 SECONDS TO ELECTROPLATE COPPER ON SAID MEMBER. THEREAFTER A DEPLATING ELECTRIC CURRENT IS CAUSED TO FLOW THROUGH SAID MEMBER FOR A PERIOD OF 6-20 SECONDS TO DEPLATE A PART OF THE COPPER PLATED ON THE MEMBER. ALTERNATE PLATING AND DEPLATING CYCLES ARE CONTINUED UNTIL A DESIRED THICKNESS OF COPPER HAS BEEN DEPOSITED ON THE MEMBER.

1974 L. M. WEISENBERGER ET AL 3,790,451

ELEGTRODEPOSITION OF COPPER FROM SULFUR-FREE CYANIDE ELECTROLYTES USING PERIODIC REVERSE CURRENT Filed Aug. 29, 1969 gaaw o INVENTORS 9A// 7 731:7 7 k [ma aw/'2' h/ss/vazmem JUL/oZB/0/cw4 ATTORNEY 3,790,451 ELECTRODEPOSITION OF COPPER FROM SUL- FUR-FREE CYANIDE ELECTROLYTES USING PERIODIC REVERSE CURRENT Lavern M. Weisenberger, Walled Lake, and Julio L.

Biora, Highland Park, Mich., assignors to The Richardson Chemical Company (Allied-Kelite Products Division), Baltimore, Md.

Filed Aug. 29, 1969, Ser. No. 854,094 Int. Cl. C23b 5/48 U.S. Cl. 204-15 14 Claims ABSTRACT OF THE DISCLOSURE A process for electroplating copper on a base member comprising applying to the member an essentially sulfurfree aqueous alkaline cyanide plating bath comprising a source of copper, an acetylenic alcohol, a complexing agent and a hydroxy acid. Current is caused to flow through the member while in contact with the plating bath for a period of 20-60 seconds to electroplate copper on said member. Thereafter a deplating electric current is caused to flow through said member for a period of 620 seconds to deplate a part of the copper plated on the member. Alternate plating and deplating cycles are continued until a desired thickness of copper has been deposited on the member.

The present invention relates to electrodepositing copper from an aqueous alkaline cyanide copper plating bath by means of a periodically reversed electric current.

Heretofore, the electrodeposition of metals by means of a periodically reversed electric current such as described in US. Pats. 2,451,341; 2,495,668; 2,636,850 and 2,678,909 has depended to a large extent on not only the length of time for each of the plating and deplating cycles and the relationship therebetween, but also on the nature as on the one hand, the presence of organic addition agents of the aqueous alkaline cyanide bath employed insofar in the baths are to be avoided to prevent blackening of the metal surfaces, and on the other hand the presence of organic sulfur compounds are to be included in the bath to produce suitably thick coatings without sacrifice of the luster characteristics thereof.

It has been found that improved copper electrodeposits can be achieved using a periodically reversed electric current technique from an essentially sulfur-free aqueous alkaline cyanide copper bath Which includes organic addition agents.

The nature of the improvement achieved by the present invention resides principally in improved levelling and corrosion resistance characteristics of the copper deposit as well as the distribution characteristics thereof which is of significant importance in certain applications of the present nvention.

The present invention has been found to be advantageously employed in Zinc die casting operations, in steel plating operations and in the production of printed circuit boards. Improved levelling characteristics have been observed, for instance, when the bath of the present invention is employed using a periodically reverse electric current technique to deposit copper on zinc die castings when compared, for instance, to the degree of levelling attained using an alkaline cyanide copper plating bath incorporating inorganic additives only, while at the same time achieving this remarkable result at considerably less sacrificial plating cycles. Additionally, conventional plating speeds have been maintained in the practice of the present invention without requiring unduly high current densities. Further, it has also been found that the bath of the present invention unexpectedly produces greater levelling United States Patent 0 characteristics when used with a periodically reversed electric current method as opposed to a direct plating procedure.

While achieving highly desirable levelling effects, the present invention also unexpectedly attains significant increases in corrosion resistance characteristics of the copper deposit. Thus it has been found that the degree of corrosion resistance attainable with aqueous alkaline cyanide copper baths not formulated in accordance with the present invention falls considerably short of the corrosion resistance characteristics attainable in the practice of the present invention, especially where, as in the automobile industry, such copper plated materials are provided with overplates of nickel and chromium. In such cases, the degree of corrosion resistance attainable with the present invention permits a significant reduction in the amount of nickel plate heretofore required to achieve comparable levels of corrosion resistance for these copper-nickelchromium composite coatings and without significantly increasing the total deposit thickness, thus alfording the practitioner considerable savings in materials.

Additionally, it has been observed that the present invention produces an improved color in the copper deposit and eliminates, or substantially reduces, the need for mechanical finishing techniques heretofore required to prepare the copper plated base material for the reception of subsequently applied metal deposits.

The present invention finds significant application in the production of printed circuit boards and especially in the field of producing through hole connections which are required to establish an electrical connection from the circuit made by the conductor on one side of the board to the conductor on the other side of a non-conductive board. Heretofore, it has been conventional to use rivets or eyelets to produce these connections. An improvement on this technique involved electroplating a conductor on the surface of the through holes which then permitted the production of the entire circuit including the conductive outer surfaces on the board as well as the through hole connections to be made by electroplating techniques.

Notwithstanding this improvement in the fabrication of printed circuit boards, it has been found that conventional electroplating techniques which involve direct plating operations, results in inferior electrodeposited copper values at the surface of the through holes. The inferiority of these deposits is based principally on a poor distribution of the copper deposit over the surface of the through hole being electroplated, i.e., the deposit exhibits significant unevenness or substantial non-uniformity of thickness which produces poor electrical properties.

The advantages of the present invention can be attained by electroplating copper from an essentially sulfur-free, aqueous alkaline cyanide copper bath containing a mixture of certain acetylenic alcohols, a hydroxy acid and a complexing agent by a periodically reversed current in which the plating period ranges from 20-60 seconds and the deplating cycle ranges from 6 to 20 seconds.

The prefered acetylenic alcohol is Z-butynediol 1,4 although other acetylenic alcohols such as S-methyl-lbutyne-B-ol, 3-methyl-1-pentyne 3 o1, l-ethynylcyclo hexanol, phenyl-propynol and 3-phenyl-l-butyne-3-ol can also be employed.

The hydroxy acid can be a sugar heptonic acid such as u-heptagluconic acid, fl-heptagluconic acid, a-mannoheptonic acid, m-galaheptonic acid, fl-galaheptonic acid, fructoheptonic acid and rhamnoheptonic acid as well as other hydroxy-acids such as tartaric acid, gluconic acid glycolic acid and saccharic acid. Generally the hydroxy acid is employed in the form of its water soluble alkali or alkaline earth metal salt and, preferably, the sodium and potassium salts thereof.

The amine type complexing agent which can usefully be employed can be ethylenediamine tetracetic acid and the water soluble salts thereof such as its sodium salt, mono-, diand triethanolamine and the like.

The pH of the bath is maintained generally between 11 to 14 and to this end the bath can contain, for instance, an alkali hydroxide such as sodium or potassium hydroxide.

The alkali cyanide copper plating bath can be that used in standard operation other than for the above combined additives. Such baths contain, in addition to copper, an alkali metal cyanide. Typical ranges of materials and conditions are:

In the following examples, the base member to be plated is immersed in the designated aqueous alkaline copper cyanide plating baths and has applied to it a plating electric current to render it a cathode, to plate copper them: from the bath for a designated period. After an increment of copper has been plated on the base member, a deplating electric current is applied to the member to render it an anode thereby removing a part of the previously plated copper. The deplating current is applied for the period designated in the specific examples. Repetition of the plating and deplating cycles continues until the desired thickness of a smooth, uni- The above plating bath formulation was employed to plate copper onto a steel member in three different runs operated in accordance with the following conditions:

Ratio of depleting current to plating current coulombs Direct plating:

Current desnity, a.s.t- Time (per cycle) Reverse plating:

Current density, a.s.L.

Time (per cycle). Total plating time, min- Bath;

Temperature, F. pH

The thickness of the copper electrodeposit achieved in each run on the steel substrate was about 0.001 inch. The percent increase in the levelling characteristics of the plated substrate was determined at these ratios of coulombs of deplating current to the plating current coulombs and is graphically shown in the drawing. The steel panel plated at an 0.0 ratio exhibited in a 42% decrease in levelling properties compared with the original steel panel. At a ratio of 0.33 the levelling characteristics of the original steel panel was improved 38% while at a ratio of 0.6 the levelling characteristics improved The bath was maintained at a temperature of about F. to F. and at a pH of about 13.5. Copper was plated from the bath on a steel panel by applying the following periodic reverse current cycle:

Plating period:

Seconds 60 Current density "amps/ft?" 510 Deplating period:

Seconds 2.0

Current density "amps/ft? 50 A second bath, identical to the above but without any 2-butyne 1,4 diol was employed under the same conditions as above to plate copper onto another steel panel. In each instance the copper was electrodeposited on the steel members for 20 minutes. The steel member plated with the bath composition containing Z-butyne 1,4 diol exhibited a 16% improvement in levelling characteristics over the steel member plated with copper in the acetylenic alcohol free bath.

EXAMPLE 3 Bath: Amount (grams/liter) Copper cyanide 63 Potassium cyanide:

(Total) 103 (Free) 11.5 Potasium hydroxide 15 Sodium gluconate 10 Z-butyne 1,4 diol 1.0

This bath was employed under the conditions recited in Example 2 to plate coper on a steel panel. For comparison, another bath, identical to the one above except that no acetylenic alcohol was included therein, was employed, again under the same operating conditions to plate copper on another steel panel. The electrodeposit of copper obtained from the bath containing the acetylenic alcohol exhibited an improvement in levelling characteristics of about 25% compared to that achieved with the use of the acetylenic alcohol free bath.

EXAMPLE 4 Bath: Amount (grams/liter) Copper cyanide 63 Potassium cyanide:

(Total) 103 (Free) 11.5 Potasium hydroxide 15 Potassium glycolate 10 2-butyne 1,4 diol 1 Again, this bath as well as another bath of essentially the same composition but for the presence of the acetylenic alcohol, were employed under substantially the same operating conditions outlined in Example 2 to produce copper electrodeposits on steel members. The copper electrodeposit achieved with the acetylenic alcohol containing bath showed a. 20% improvement in levelling characteristics over the copper electrodeposit attained with the use of the acetylenic alcohol free bath.

This bath as well as another bath of essentially the same composition except for the inclusion therein of the 2-butyne 1-4, diol were employed under essentially the same plating conditions set forth in Example 2 to produce a copper deposit on steel members. A 20% improvement in the levelling properties of the copper electrodeposit produced using the acetylenic alcohol containing bath over the levelling properties of the copper plate achieved using the acetylenic alcohol free bath was noted.

EXAMPLE 6 Bath:' Amounts (grams/liter) Copper cyanide 63 Potassium cyanide:

(Total) 103 (Free) 11.5 Potassium hydroxide 15 Potassium glucoheptonate 2-butyne 1-4, diol 1 A similar bath but for the inclusion therein of any acetylenic alcohol was also prepared and both baths were used in accordance with the operating procedures set forth in Example 2 to produce a copper electrodeposit on steel panels. The electrodeposit achieved from the bath containing the acetylenic alcohol showed a 25% improvement in levelling properties over the electrodeposit from the acetylenic alcohol free bath.

An essentially identical bath but without the acetylenic alcohol was also prepared and both baths were used to electrodeposit copper on steel panels under the operating conditions of Example 2. The electrodeposit achieved using the bath containing the acetylenic alcohol exhibited a 65% improvement in levelling properties over the electrodeposit produced from the acetylenic alcohol free bath.

EXAMPLE 8 To the bath composition of Example 7 containing an acetylenic alcohol there was added 9 grams/liter of sodium thiocyanate and using this bath, again in accordance with the conditions set forth in Example 2, copper was plated onto a steel member. The levelling characteristics of the copper electrodeposit achieved showed a 25-30% reduction compared to those achieved using the acetylenic alcohol containing bath composition of Example 7.

Additionally tests were conducted to demonstrate the improved corrosion resistance characteristics obtainable with the present invention over systems not employing the present inventive principles. Thus, using a bath formulated as follows:

Bath: Amounts (grams/liter) Copper cyanide 63 Potassium cyanide:

(Total) 103 (Free) 11.5 Potassium hydroxide 15 (Hydroxy acid) 4.2

(Amine complexing agent) 0.35 (Acetylenic alcohol) 1.0

Zinc die castings were plated for a total of 17 minutes with copper using periodic reverse current cycle as follows:

Plating period:

Seconds 38 Current density "amps/ft?" 40 Deplating period:

Seconds 14 Current density "amps/ft?" 30 A second copper cyanide bath having the following composition was also employed to electroplate zinc die castings for a total of 17 minutes by a periodic reverse current cycle:

Bath- Amounts (grams/liter) Copper (as a metal) 45, Potassium hydroxide 15' Free potassium cyanide 11.5 Zinc oxide 1.0, Metal thiocyanate 5.9

The periodic reverse current cycle employed was as follows:

Plating period:

The copper plated zinc dies castings resulting from both procedures were tested to determine their corrosion resistance characteristics and it was found that those produced in accordance with the present invention gave a rating of 92 or better on a conventionally accepted scale, the testing procedures being performed in accordance with ASTM B368-65 (Copper Accelerated Acetic Acid-Salt Spray (Fog) Testing-CASS test). The zinc die castings which were copper plated with the zinc oxide-metal thiocyanate bath outlined above, when similarly tested, failed to give acceptable ratings, 92 being the minimum rating for acceptable corrosion resistance characteristics.

The present invention is useful in the production of printed circuit boards which, of course, enjoy wide usage in the electrical and electronic industries because of the case of wiring and low cost circuit connections resulting from using such boards. In order that electrical connections may be established from the circuit made by the conductor on one side of the board to the conductor on the other side of the board, it is common practice to form holes through the board and to conductively connect these conductors through these holes. The conductor surfaces of the printed circuit board can be provided on the opposed surfaces of the dielectric base material in any conventional manner with the through holes being plated in accordance with the present invention or in, the alternative, these conductor surfaces as well as the through holes can be plated in accordance with the present invention.

Conventional methods to treat the surfaces of a dielectric base material to render the same receptive to the copper electro-deposit in accordance with the present invention include (1) placing a seeding film of metal catalytic to chemical reduction plating on the dielectric base material and chemically plating a conductive metal over the seeding layer; (2) painting a conductive metal powder in an organic vehicle on the surfaces of the dielectric base material to be electroplated; (3) applying graphite to the surfaces of the dielectric base material to be electroplated and (4) depositing a thin film of a conductive metal, such as palladium in colloidal form on the surfaces of the base material to be electroplated.

Preferably the treated dielectric base is provided with a substantially continuous protective metal coating, having generally a thickness ranging up to about 0.2 mil thereby avoiding any deleterious effects such as blistering or attack of the board by the subsequent contact thereof by the aqueous alkaline cyanide electroplating bath. Conveniently, it has been found that this protective metal coating can be deposited on the treated dielectric base member from a copper pyrophosphate bath or any other conventional aqueous metal bath solution. Of course, metals other than copper can be employed, for instance, nickel and the like.

Thereafter, the base is electroplated by immersing the same in an aqueous alkaline copper cyanide bath as described above using a periodically reversed electric current operation as described above.

EXAMPLE 9 A printed circuit board base material was cut to the desired size and holes drilled therein at the desired places. The surfaces were roughened by a vapor blast and then cleaned to remove the grit produced by the vapor blast. The surfaces are thereafter sensitized by treating the same with 12 g./l. solution of stannous chloride and subsequently with a 1.85 g./l. solution of palladium chloride to deposit on the surfaces of the base material a layer of palladium which is catalytic to chemical reduction plating techniques. Thereafter, the printed circuit board can be provided with an electroless flash coating of copper by immersing the same in an aqueous chemical reduction plating bath containing a source of copper and a reducing agent such as sodium hypophosphite to deposit a thin layer of flash coating of copper on the printed circuit base material. Obviously any other conventional method of rendering the surfaces of the printed circuit base material receptive to the copper electrodeposit of this invention can be used.

A resist masking was then placed on the board to define the pattern of the desired circuit. Then the unmasked portion of the flashed copper coated board was provided with a protective coating of copper which was electrodeposited frrom an agitated copper pyrophosphate bath containing 3.5 g./l. of metallic copper, the ratio of total pyrophosphate to copper metal being 7.2:1.3 g./l. of NH OH (29% solution). The pH of the bath was maintained at about 8 and a temperature of 140 F. Thereafter the treated and protected board was provided with an electrodeposit of copper in accordance with the present invention using an aqueous alkaline copper cyanide plating bath having the following composition:

Ethylenediamine tetraacetic acid, sodium salt 0.35

The bath was maintained at a temperature of about 150 F. and a pH of 13.5. Copper was plated on the circuit board for a total of 28 minutes using a periodic reverse current cycle as follows:

Plating period:

Seconds 60 Current density Amps/ft?" 40 Deplating period:

Seconds 20 Current density Amps/ft. 40

The resist is then removed from the board and the electrodeposited copper which defines the circuit is protected while the copper flash coating on the outer surfaces is removed by conventional etching techniques.

The present invention can also be used to provide an electrodeposit in the through holes only of a printed circuit board previously provided with conductive outer surfaces in essentially the same manner using the above periodic reverse current cycle. It will be understood that various modifications in the form and details of the operation can be made by those skilled in the art Without departing from the spirit of the invention.

What is claimed is:

1. A process for electroplating copper on a base member comprising applying to the member an essentially sulfur-free aqueous alkaline cyanide plating bath comprising a source of copper, an acetylenic alcohol, a complexing agent and a hydroxy acid, causing a plating current to flow through the member while in contact with the plating bath for a period of 20-60 seconds to electroplate copper on said member, then causing a deplating current to flow through said member for a period of 6-20 seconds to deplate a part of the copper plated on the member and continuing alternate plating and deplating until a desired thickness of copper has been deposited on the member.

2. The process of claim 1 wherein the essentially sulfurfree aqueous alkaline cyanide plating bath contains 30- g./l. copper cyanide, 45 to g./l. alkali metal cyanide selected from the group consisting of sodium and potassium cyanide, 0.5 to 10 g./l. acetylenic alcohol, 0 to 20 g./l. complexing agent and 2 to 50 g./l. hydroxy acid or water soluble salt thereof.

3. The process of claim 2 wherein the acetylenic alcohol is selected from the group consisting of 2-butyne 1-4, diol, 3-methyl 1-butyne-3-ol, 3-methyl-l-pentyne-3-ol, 1- ethynylcyclohexanol, phenyl-propynol and 3-ph'enyl-1- butyne-3-ol.

4. The process of claim 2 wherein the hydroxy acid is selected from the group consisting of sugar heptonic acid, tartaric acid, gluconic acid, glycolic acid, saccharic acid and the water soluble salts thereof.

5. The process of claim 2 wherein the complexing agent is selected from the group consisting of ethylenediamine tetraacetic acid and the water-soluble salts thereof and mono-, di-, and triethanolamine.

6. The process of claim 2 wherein the current density during the plating cycle ranges between 10 to 100 amps/ft. and the current density during the deplating cycle ranges between 5 to 100 amps/ft).

7. A process for electroplating copper on a base member comprising applying to the member an essentially sulfur-free aqueous alkaline cyanide bath comprising 30- 100 g./l. copper cyanide, 45-150 g./l. alkali metal cyanide, 0.5-l0 g./l. acetylenic alcohol, 0-20 g./l. complexing agent, and 2-50 g./l. hydroxy acid or Water soluble salt thereof, causing a plating current to flow through the member while in contact with the plating bath for a period of 20-60 seconds to electroplate copper on said member, the current density during the plating cycle ranging between 10-100 amps/ft then causing a deplating current to flow through said member for a period of 6-20 seconds to deplate a part of the copper on the member, the current density during the deplating cycle ranging between 5-100 amps/ft, and continuing alternate plating and deplating 9 until a desired thickness of copper has been deposited on the member.

8. A method for producing a conductive copper coating on through holes in printed circuit boards comprising treating the surface of said through holes to render the same receptive to a copper deposit thereon and electroplating copper on said treated surface by applying to the surface an essentially sulfur-free aqueous alkaline cyanide plating bath comprising a source of copper, an acetylenic alcohol, a complexing agent and a hydroxy acid, causing a plating current to fiow through the surface while in contact with the plating bath for a period of 60 seconds to electroplate copper on said surface, then causing a deplating current to flow through said surface for a period of 620 seconds to deplate a part of the copper plated on the surface and continuing alternate plating and deplating until a desired thickness of copper has been deposited on the surface.

9. The process of claim 8 wherein the essentially sulfurfree aqueous alkaline cyanide plating bath contains 100 g./l. copper cyanide,: to 150 g./l. alkali metal cyanide, 0.5 to 10 g./l. acetylenic alcohol, 0 to 20 g./l. complexing agent and 2 to g./l. hydroxy acid or water soluble salt thereof.

10. The process of claim 9 wherein the alkali metal cyanide is sodium or potassium cyanide.

11. The process of claim 9 wherein the acetylenic alcohol is selected from the group consisting of 2-butyne l-4, diol, 3-methyl 1-butyne-3-ol, 3-methyl-1-pentyne-3-ol, 1-

10 ethynylcyclohexanol, phenyl-propynol and 3-phenyl-lbutyne-3-ol.

12. The process of claim 9 wherein the hydroxy acid is selected from the group consisting of sugar heptonic acid, tartaric acid, gluconic acid, glycolic acid, saccharic acid and the water soluble salts thereof.

13. The process of claim 9 wherein the complexing agent is selected from the group consisting of ethylenediamine tetraacetic acid and the water-soluble salts thereof and mono-, di-, and triethanolamine.

14. The process of claim 9 wherein the current density during the plating cycle ranges between 10 to amps/ ft. and the current density during the deplating cycle ranges between 5 to 100 amps/ft.

References Cited UNITED STATES PATENTS 2,636,850 4/1953 Jernstedt et a1 20452.1 3,084,112 4/1963 Leenders et a1. 20452.1

FOREIGN PATENTS 598,028 5/1960 Canada 20452.1

JOHN H. MACK, Primary Examiner W. I. SOLOMON, Assistant Examiner US. Cl. X.R.

204-35 R, 52 Y, Dig. 9

E UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 7 Dated February 5, 97

'Inventor(s) Lavern 1 1. Weisenberger and Julio L. Biora It is certified that error appears in the above-identified patent 'and that said Letters Patent are hereby corrected as shown below:

Column 1, after the words in line3 8,it should read as follows:

"of the aqueous alkaline cyanide bath employed insofar as on the one hand, the presence of organic addition agents in the baths are to be avoided to prevent blackening of the metal surfaces, and on the other hand the presence of organic sulfur compounds are to be included in the bath to produce suitably thick coatings without sacrifice of the luster characteristics thereof." a

Column 1, line 56 "nvention" should be "invention" Column 1, line 62 "reverse" should be "reversed" Column T, line 5" 't'frrom" should be "from" Signed sealed this 31st day of December 1974'.

(SEAL) Attest:

McCOY M. GIBSON JR. C. T'EARSHALL DAN Attesting Offfmer Commissioner of Patents F ORM PO-105O (10-69) THE PENGAD COMPANIES, INC ATTORNEYS SUPPLIES DI 55 OAK STREET, BAYONNE. NJ. 01002 (20H 486'5825 

